This invention relates generally to mattress foundation structures and more particularly to a box spring assembly having a frame and springs having mounting feet secured to the frame.
A box spring assembly typically has a horizontal frame which is rectangular in shape. Spaced above and generally parallel to the frame is a mattress support deck made of wire. A plurality of deck support spring modules are interposed between the frame and the wire deck to yieldably resist bedding loads on the box spring assembly. The spring modules generally have deck attaching portions at the upper end and mounting foot portions at the lower end of the modules.
The rectangular frame typically has metal or wood rails of a type shown in U.S. Pat. Nos. 3,680,157 and 3,755, 833. The typical mounting foot of a spring module disclosed in these patents is either stapled to the wood frame rail member or inserted through horizontal slots in the metal frame rail members. These prior art mounting feet are generally U-shaped horizontal wire portions at the bottom end of the spring module.
Each foot comsists of a pair of spaced legs spring urged apart. One straight leg is connected to the upright yieldable portion of the spring module and forms a horizontal torsin bar which frictionally rubs against one end of the slots in the rail member. The other leg is connected to the first leg by a connecting portion and terminates in a book shaped free end. The hook shaped end straddles one end of one slot in the frame rail to prevent dislocation of the mounting foot from the slots in the frame rail member. This leg also urges the other leg into engagement with the ends of the slot to provide firm engagement of the mounting foot within the slots.
One disadvantage of spring modules having a mounting foot structure according to these prior art designs is that the hook portion is the primary portion of the U-shaped foot that retains the foot within the slot. Thus the retention forces under loaded conditions tend to concentrate at the hooked portion.
Another disadvantage in the prior art designs is that horizontal twisting loads and impact loads applied to the spring modules tend to dislodge the straight torsion bar leg of the mounting foot from the slot ends. Under heavy shock loading, the mounting foot in spring modules of the prior art designs may shift out of the engaged position with the slot ends without properly realigning and remain in an unbalanced condition. This can generate undesirable noise.
It is therefore an object of the present invention to provide a mounting foot structure which evenly distributes the retention forces present between both legs of the mounting foot, thus more evenly distributing wear and more firmly securing the foot within the frame rail member.
It is another object of the present invention to provide a mounting foot structure having a retaining portion on the torsion bar leg to improve the retention characteristics of the foot under horizontal twisting loads and impact loading conditions.
A spring module for a box spring assembly according to the present invention is designed for use with a box spring frame rail having horizontally spaced generally upright side walls joined by a horizontal connecting portion forming an inverted U-shaped channel member. The frame rail has a plurality of aligned pairs of slots through the upright side walls spaced along the length of the rail. Each slot has a central portion and opposing closed ends which form end bearing portions of substantially C-shape. Such a slotted frame rail is shown in copending application Ser. No. 72,964, filed July 14, 1987, Pat. No. 4,779,292 assigned to the assignee of this application and the disclosure of which is incorporated herein by reference.
The spring module has an upright yieldable portion supported on the rail member by a generally U-shaped horizontal mounting foot at the lower end of the upright yieldable portion. The mounting foot is a generally U-shaped wire portion of the spring module comprising a pair of spaced leg portions joined together by a connecting portion which spring urges the leg portions apart. Each of the leg portions has a pair of spaced bearing sections that are straight for engaging the end bearing portions at one end of one pair of slots in the side walls of the frame rail. Between the bearing sections, each leg portion has an outwardly convex retaining portion which coacts with the side walls to retain the leg portion in the slots. As the leg portions are spring urged apart by the connecting portion, the two outwardly convex retaining portions distribute the retention forces during twisting of the spring module to firmly retain the mounting foot within slots in the frame rail member.
The combination of a pair of spaced bearing sections separated by an outwardly convex retaining portion minimizes the amount of friction between the bearing sections of the leg portions and the end bearing portions of the slots during torsional movement of the leg portion connected to the upright yieldable portion of the spring module. In addition, the improved mounting foot utilizing a pair of spaced straight bearing sections balances the mounting forces so that equal loading is maintained on both ends of the slots in the frame rails while the foot is maintained in a fixed position. Any rotational forces tending to move the foot out of position are deflected against the upright walls of the frame rails thus eliminating movement and therefore noise. The spring module having an improved mounting foot structure in accordance with the present invention also has a longer installed life and is more stable under shock and impact loading.
Other objects, features and advantages of the present invention will become apparent from a consideration of the following description and the appended claims when taken in conjunction with the accompanying drawing.